Although polyesters in general and poly(ethylene terephthalate) in particular have fairly good stability towards sunlight, long term exposure to sunlight results in a loss of desirable physical properties, for example when tested for embrittlement, yellowing, tensile strength, impact strength, flexural strength, gloss, and solution viscosity.
It is well known that organic polymers can be rendered resistant to degradation by ultraviolet (UV) light by physically blending such polymers with various UV light absorbing compounds such as benzophenones, benzotriazoles, resorcinol monobenzoates and certain 4-oxybenzylidene compounds. The admixed UV absorbers have the disadvantage of having a tendency to sublime under high temperature processing conditions, thus resulting in the loss of the UV absorber and contamination of processing equipment. Also, the UV absorbers may be extractable or may exude from the polyesters under certain conditions.
It is known that some improvement in photostability and weatherability can be achieved by replacing some of the terephthalate residues by 2,6-naphthalene dicarboxylate residues. (See, for example, B. Milligan, Rev. Prog. Coloration, Vol. 16, 1986, pp. 1-17; and P.S.R. Cheung, et al., J. Appl. Polym. Sci., 2.4, 1809 (1979). It is also known that the incorporation of certain 4-oxybenzylidene moieties into polyesters improves weatherability and photostability. (See, for example, U.S. Pat. Nos. 4,338,247 and 4,340,718.) The present invention, as described below, provides a copolyester comprised of both 2,6-naphthalene dicarboxylic acid and 4-oxybenzylidene moieties having improved weatherability.